This invention relates to a method of and an apparatus for producing a pillar-like metal tip, especially for use as an electrode of a spark plug or the like. The invention also relates to a spark plug employing such a metal tip.
As shown in FIG. 14, a spark plug 90 for internal combustion engines includes a central electrode 92 provided in an insulator 91, and an earth electrode 93 provided at a lower end portion of a housing 95. A type of spark plug is known in which a metal tip is used at a discharge end 94 of the central electrode 92 or the earth electrode 93.
This metal tip must be resistant resistance to a discharge and a heat resistance. In many cases, noble metal such as platinum is used for forming the metal tip. In FIG. 14, the reference numeral 951 denotes a mounting threaded portion, the reference numeral 952 a central stem, the reference numeral 953 a ring, the reference numeral 954 a packing, and the reference numeral 955 an electrically-conductive glass seal material.
In order to decrease a voltage (hereinafter referred to as "required voltage") needed for initiating a spark discharge, and also to enhance the ignitability of a fuel mixture, it has been proposed to form a groove-like recess in a distal end of the metal tip (see Japanese Patent Unexamined Publication No. 55-19768, Japanese Patent Examined Publication No. 59-33949 and Japanese Patent Unexamined Publication No. 3-225783).
More specifically, such a metal tip 941 has a recess 942 at its central portion, and flat projection surface portions 943 are provided immediately adjacent to the recess 942, and a connecting portion 944 having a thickness t is provided at a bottom thereof, as shown in FIG. 15.
The metal tip 941 is joined or welded to the distal end of the central electrode 92 to serve as the discharge end 94 of the central electrode 92. When the spark plug is used for a long period of time, the projection surface portions 943 of the metal tip 941 are worn down by the spark discharge, as indicated by broken lines in FIG. 16.
As a result, as seen in FIG. 16, the discharge gap increases from G.sub.1 to G.sub.2, so that the required voltage increases. When the lifetime of the spark plug expires, the spark plug, including the metal tip 941 made from noble metal or the like, is discarded.
Japanese Patent Unexamined Publication No. 55-19768 and Japanese Patent Examined Publication No. 59-33949 disclose a method of producing a pillar-like metal tip of the type described having a recess, in which the groove-like recess is formed by cutting or severing.
With such a method, however, an unsatisfactory straightness of the central electrode, displacement of a chuck for holding the central electrode, and displacement of a cutting tool jointly contribute to irregularities in the position of formation of the groove-like recess 942 and in dimensions of the groove, so that the projection surface portions 943 vary in size.
As a result, the spark discharge concentrates mainly on the projection surface portion 943 having a smaller area, and an electrode pole having this projection surface portion 943 with a smaller area is worn and exhausted. Thus, the number of discharge poles decreases, and the effect of decreasing the required voltage, which the above Japanese Patent Examined Publication No. 59-33949 seeks to achieve, is adversely affected.
Furthermore, due to variations in the position and dimension of the groove-like recess 942, that area of the discharge end 94 of the central electrode which serves to achieve a quenching effect is also varied, so that the ignitability of a fuel mixture, which the above Japanese Patent Unexamined Publication No. 55-19768 seeks to achieve, is varied.
The above Japanese Patent Unexamined Publication No. 3-225783 discloses a method in which a noble metal tip is joined to a distal end of a central electrode, and then a groove-like recess is formed in this metal tip by cutting. In this case, a problem is encountered in that the expensive noble metal is wasted in the form of cuttings or chips. Even if recovery of the cuttings of the noble metal is attempted, this is difficult because such cuttings are mixed with cuttings of other electrode materials.
Moreover, the method of forming the recess in the metal tip by cutting is not satisfactory in that much time and labor are required, thus increasing the cost. More specifically, much time and labor are required because the feeding speed of the cutting tool is low and also because an additional step of holding the central electrode with a chuck is required before the cutting operation.
Another problem is that the recess formed by cutting is limited to a straight groove-shape, and therefore the cross-sectional shape of the projection surface portion 943 is very limited, and hence is not always the optimum one. Here, the optimum cross-sectional shape means a shape best suited for enhancing the ignitability of a fuel mixture, the required voltage characteristic, and the lifetime of the electrode.
Apart from the above processing method depending on the cutting operation, a processing method is known depending on cold forging, in which processed dimensions are stable, less time and labor are required, and materials or blanks are not wasted.
One example of method of producing a metal tip with a recess by cold forging is a forward-extruding method shown in FIG. 17, in which a projection 962 corresponding in shape to a desired recess is formed on a bottom surface 961 of a die 96, and a metal blank 97 is pressed by a punch 980 from the upper side, so that the metal blank 97 is extruded in a direction of advance of the punch 980.
Another example is a rearward-extruding method shown in FIG. 18, in which a punch 981 having a projection 982 corresponding in shape to a recess is pressed against a metal blank 97, so that the metal blank 97 is plastically flowed in a direction opposite to the direction of advance of the punch 981.
However, these cold forging methods have the following problems:
In the latter method, (that is, the rearward-extruding method), as the punch 981 advances, the metal blank 97 is flowed and deformed by the punch 981. In accordance with the flow and deformation of the metal blank 97, the punch 981 receives a large repulsion force from those portions of the metal blank which are not subjected to such flow and deformation.
The connecting portion (as at 944 in FIG. 15) of the metal tip should preferably be small in thickness t for reasons later described. But when, the thickness t of this connecting portion is made small, a frictional force between a die 96 and the punch 981 due to the flow and deformation of the metal blank 97 becomes extremely large. As a result, the punch 981 is liable to be broken.
For the same reason, in the former method, (that is, the forward-extruding method), the projection 962 of the die 96 is susceptible to breakage.
The hardness of the metal blank 97 corresponds to the likelihood that the projection 962 of the die 96 and the projection 982 of the punch 981 can be broken. The metal tip used for an electrode of a spark plug is made of a hard material having a Vickers hardness of 200.about.300, such as a heat resisting Ni alloy and a noble metal alloy.
Since an expensive material, such as a noble metal, is used for a spark plug, the thickness t (FIG. 15) of the connecting portion should be as small as possible in order to save the material. Therefore, the thickness t of the connecting portion is typically not more than 0.3 mm, so when producing the metal tip by cold forging, the projection of the punch or the die is quite susceptible to breakage.